Alpha, beta, gamma, and delta tocopherol (hereafter referred to as tocopherols) can be found in various ratios and concentrations in crude vegetable oils such as soy bean, sunflower, canola, rapeseed, cottonseed, safflower, corn, palm, palm kernel, and rice bran oil. Palm oil and rice bran oil in particular contain high levels of both tocopherols and tocotrienols, whereas other vegetable oils contain primarily tocopherols. Typical crude palm oils contain 600-700 mg/Kg tocopherols and tocotrienols (50% tocotrienols) and crude rice bran oil contains 800-900 mg/Kg tocopherols and tocotrienols (57% tocotrienols). (See, for example, Proc. Malays. Biochem. Soc. Conf. (1983). pp. 15-17 and Lipids Handbook, 2d Ed., (1994) Chapman & Hall, p. 129.) Tocopherols and tocotrienols are a valuable constituent of vegetable oil as they help prevent oxidation and spoilage. Tocotrienols are of special interest for their hypocholesterolemic effects, as they decrease the blood level of the low density lipoprotein fraction of cholesterol and the total serum cholesterol, while increasing the ratio of the high density lipoprotein fraction of cholesterol to the low density lipoprotein fraction. Such effects have been shown to be clinically significant in lowering the risk of heart disease. (T. Gordon, et al., "High Density Lipoproteins as a Protective Factor Against Coronary Heart Disease", The American Journal of Medicine, 62, pp. 707-714 (1977)). During the refining of vegetable oils a large fraction of the tocopherols are lost to various by-products and waste streams. These waste and by-products streams include, but are not limited to, deodorizer distillates, steam refining distillates, and acidulated soapstocks. The vegetable oil refining by-products typically contain from less than 1% to greater than 20% tocopherol by weight. Such by-products from the tocotrienol-enriched vegetable oils such as rice bran oil and palm oil generally contain about 0.1 to 5% by weight of tocotrienols. The oil refining by-products are a valuable source of raw material for the production of natural vitamin E and other tocopherol antioxidants. However, the by-product streams also contain 20 to 99% by weight free fatty acids, less than 1% to 20% by weight sterols, less than 1% to 20% by weight sterol esters of fatty acid, less than 1% to 40% by weight mono, di, and triglycerides, less than 1% to 30% by weight hydrocarbons, and several percentage by weight of other compounds, in addition to tocopherols and tocotrienols. Thus, in order to obtain a tocopherol/tocotrienol concentrate stream useful for production of high purity vitamin E, it is necessary to remove these substances.
Numerous methods have been proposed for the recovery of tocopherols from vegetable oil refining by-products. For example, U.S. Pat. No. 2,432,181 teaches that tocopherols can be recovered from vegetable oils and fats by reacting the fatty acid glycerides with an aliphatic monohydric alcohol in the presence of an alkaline alcoholysis catalyst, followed by a flash distillation of residual alcohol glycerol, and fatty acid esters.
U.S. Pat. No. 3,153,055 teaches a process for the isolation of sterols and tocopherols from deodorizer distillate by esterification of free fatty acids and glycerides into lower monohydric alcohol esters under strongly acidic conditions. The sterols and tocopherols are fractionally extracted from the esterification product with a combination of polar and nonpolar solvents.
U.S. Pat. No. 3,335,154 teaches that deodorizer distillate can be saponified and acidulated to convert glycerides and sterol esters to free fatty acids and free alcohols (glycerol, sterols respectively). The free fatty acids are esterified with a monohydric lower alcohol and mineral acid catalyst. The sterols are precipitated/crystallized by the addition of water to the mixture, and the tocopherols are concentrated by removal of the fatty acid esters by molecular distillation.
All of the above processes suffer from serious drawbacks. They require the addition of extraneous monohydric alcohols and result in the production of fatty acid esters which are not normally present in the vegetable oil by-product feed material. The excess monohydric alcohol must be removed in an additional processing step. In order to produce a highly concentrated tocopherol product the sterols must either be removed by crystallization or by other means. Saponification requires large amounts of caustic and acid for acidulation, thereby creating excessive salt wastes.
U.S. Patent No. 4,454,329 teaches that a tocopherol concentrate can be obtained from deodorizer distillates by esterification of the free fatty acids with a dihydric or polyhydric alcohol, in the presence or absence of an acid catalyst. The esterification is preferably carried out in the presence of an aromatic solvent such as benzene, toluene, or xylene. The esterified mixture is then subjected to either a solvent extraction or a molecular distillation to produce the final tocopherol concentrate. Preferably, the solvent extraction is proceeded by a hydrogenation to convert the unsaturated triglycerides into saturated triglycerides, thereby decreasing the solubility of the triglycerides in the solvent phase of the extraction. Distillation of the esterified mixture concentrates tocopherols, sterols, hydrocarbons and other components with similar boiling points in the distillate. The triglycerides and other high-boiling components are left in the distillation residue.
The above process is unsatisfactory for a number of reasons. Deodorizer distillates and the like typically contain a 1/1 to 3/1 ratio of sterols to tocopherols, depending on the vegetable oil source. The tocopherols and sterols have very similar boiling points and therefore cannot be separated by distillation alone. The esterification is not run in such a fashion to ensure that the sterols are converted into sterol esters (which have a much higher boiling point than tocopherol). The distillate containing the tocopherols and sterols produced by the above process must be further treated by other separation techniques in order to produce a tocopherol concentrate essentially free of sterols.
In the solvent extraction version of the process, the solvent must be removed from the tocopherol extract, adding additional cost and complication to the process. The preferred embodiment of the solvent extraction, proceeded by the hydrogenation, adds still another step, with concomitant cost and complication. In addition, typical copper and nickel hydrogenation catalysts are known to be prooxidants, which promote the destruction of tocopherol, thereby lowering the yield of tocopherol from the process.